1. Field of the Invention
The present invention relates to a thin-film transistor (hereinafter referred to as TFT), formed of a non-single-crystal silicon film disposed on an insulating substrate, and a liquid crystal display device furnished with the same.
2. Description of the Related Art
Since those TFTs which use a thin non-single-crystal silicon film, such as an amorphous silicon (hereinafter referred to as a-Si:H) or polycrystalline silicon (hereinafter referred to as p-Si) film, can be formed uniformly on an insulating substrate, covering a relatively wide area, they are conventionally used in pixel switches, driver circuits, etc. of active-matrix liquid crystal display devices.
In the case where one such TFT is used as a pixel switch of an active-matrix liquid crystal display device, the electrical resistance of a gate electrode of the TFT must be lowered in order to lessen waveform distortion. Gate electrodes that meet this requirement are described in Jpn. Pat. Appln. KOKAI Publication Nos. 4-353830, 5-152572, and 6-120503. These gate electrodes are composed of a low-resistance metal, such as aluminum (Al), coated with chromium (Cr), tungsten (W), titanium (Ti), tantalum (Ta), or some other metal whose melting point is higher than that of aluminum or with an aluminum alloy so that a hillock, blister, etc. of aluminum can be effectively prevented.
In Jpn. Pat. Appln. KOKAI Publication No. 4-372934, the inventors hereof proposed a method in which molybdenum (Mo) is deposited on an aluminum film, etched with use of a mixed acid, and then removed so that the aluminum film is tapered taking advantage of the difference in etching rate, in forming a gate electrode of a laminated structure from aluminum and some other metallic material, e.g., a molybdenum-tantalum (Mo--Ta) alloy, covering the aluminum film. According to this method, an insulating film on the gate electrode can be prevented from suffering insulation failure.
In order to enjoy excellent element characteristics, it is advisable to enhance the mobility of the TFT and lower its threshold voltage (V.sub.th), in view of the operating efficiency. With the gate electrode multilayered to lower its electrical resistance, however, the TFT has a tendency to exhibit low mobility and high threshold voltage.